Measuring the effect of global warming on weather extremes

Two studies recently published by the World Weather Attribution (WWA) group attempt to measure the effect of global warming on the intensity of natural disasters. The researchers point out that the global average temperature has risen by 1.3 degrees compared to the pre-industrial era.

The first study was published at the end of September and looked at the major floods that hit a very large part of Central Europe from September 12 to 16, 2024, during the passage of storm Boris. 

Communities in Poland, the Czech Republic, Slovakia, Austria, Hungary, Romania and Germany received exceptional amounts of rainfall over four consecutive days. More than two million people were affected. At least 28 people lost their lives, according to estimates made at the end of September.

Several of the flooded regions had already suffered similar floods in 1997 (100 deaths in Germany alone) and 2002 (232 deaths). The lower number of deaths attributable to the 2024 floods is due to a number of factors: weather forecasts were more accurate, warnings were numerous and issued in sufficient time for evacuations to take place in advance. What's more, in several regions, infrastructures had been modified to better withstand the volume of rain.

The disaster was caused by a “Vb” low-pressure system. These low-pressure systems form when cold polar air flows from the north over the Alps, deflects southwards, initiating a cyclonic system over the Alps and northern Italy, which then develops and moves towards central and eastern Europe. Exceptionally cold air masses moved over the Mediterranean and Adriatic Sea a few days before the rain event, recharging with moisture before moving inland.

History repeats itself 

In Poland, over 400 mm of rain fell during this climatic event. At a webinar attended by several of the study's authors, Professor Bogdan Chojnicki, from the Department of Agrometeorology at University of Life Sciences in Poznan, stressed the need to review compensation programs. 

“Building from scratch is really not a good idea, and governments need to stop doing it. In 1997, we had floods that were estimated to happen once in a millennium. It didn't take 27 years, and we had another historic flood,” he says. He adds that weather systems are accumulating more energy as a result of warming, and that this will most likely lead to an increase in extreme events.

For her part, Friederike Otto, of the Grantham Institute – Climate Change and the Environment at Imperial College London, says that with a global warming scenario of 2°C by 2060, the probability of a similar event of four intense rainy days will be 50% higher than at present, and the volume of rain will be 5% higher.

The bill for insured damage will be 2 to 3 billion euros (G€), according to Gallagher Re's estimate reported in the WWA study. On the subject of the vulnerability of certain states to flood risk, the World Bank estimates that Germany already suffers an average of €7.9 billion a year in insured damage from fluvial and pluvial flooding. In this respect, the country could see a 25% annual increase in insured losses by 2050. 

Hurricane Helene  

World Weather Attribution published a similar study in the wake of Hurricane Helene, which devastated the southeastern United States on September 26. The storm caused enormous damage, with insured losses in the tens of billions of dollars, and claimed at least 227 lives. Helene became one of the deadliest hurricanes in 50 years, second only to Hurricane Katrina in 2005.

The storm surge reached 15 feet (4.5 meters) in Florida's Big Bend region, which had never before been hit by a Category 4 hurricane. The Big Bend region was hit by three hurricanes in 13 months, following Idalia in August 2023 and Beryl in July 2024.

The 225 km/h winds and huge amounts of rain that accompanied the hurricane devastated a vast territory stretching over 800 km, from Florida to northwest North Carolina, via Georgia, Tennessee and Virginia. 

The WWA study was made public during a webinar held on October 8, in which the Insurance Portal took part. Another major hurricane, Milton, was approaching Florida at the time. 

The facilities of the National Centers for Environmental Information, located in Asheville, North Carolina, were hard hit by the hurricane. This laboratory, part of the National Oceanic and Atmospheric Administration (NOAA), is an essential resource for climate analysis, says Bernadette Woods Placky, Chief Meteorologist at Climate Central.

She points out that Hurricane Helene went from Category 2 to Category 4 on the Saffir-Simpson scale in just 10 hours. “This acceleration in the intensity of tropical storms is becoming more frequent with climate change. If human activity continues to warm the climate, we'll see more and more storms turn into monster hurricanes that do even more damage,” she says. She adds that fossil fuel emissions add heat to the atmosphere and oceans, and these elements “are steroids for hurricanes”.

The very high ocean temperatures in the Gulf of Mexico were repeatedly cited as contributing to the intensification of the hurricane. Hurricanes of Category 3 and above are referred to as major hurricanes. Hurricane Milton struck near Sarasota at around 8 p.m. on Wednesday, October 9. 

Not a first 

Had there ever been two major hurricanes hitting Florida in less than two weeks before, Insurance Portal asked? Ms. Woods Placky points out that at the time it was not yet known how strong Milton would be when it made landfall; it was a Category 3 after all. 

The data she reports from the Florida Climate Center show that in 2004, Florida suffered four hurricanes in six weeks, including three major ones: Charley (category 4, August 13), Ivan (category 3, September 16) and Jeanne (category 3, September 26). Hurricane Frances had lost strength and was downgraded to category 2 when it hit Florida on September 2, 2004.

However, the signal of atmospheric pressure dropping to 897 millibars at the center of the hurricane, recorded as Milton gained intensity in the Gulf, was very worrying. According to Woods Placky, if the storm had made landfall on the Yucatan Peninsula, rather than bypassing it, the damage would have been considerable. 

The WWA study, which combines several climate models of weather extremes and vulnerability, reveals that hurricanes of the same intensity as Helene are 2.5 times more likely to occur in this region. Whereas such an event used to recur every 130 years, it now occurs once every 53 years.

In addition, Helene's winds were 21 km/h stronger, or 11%, than they would have been without global warming. The volume of rainfall increased by 10%. Water temperatures in the Gulf of Mexico, which contributed to the hurricane's intensity, are clearly attributable to global warming. 

Not just on the coasts 

Finally, the researchers believe that US authorities must now prepare for catastrophic flooding associated with these hurricanes not only in coastal states, but also in neighboring states, as seen in northwestern North Carolina. 

In some places in this region, more than 30 inches of rain (or 762 mm) were recorded over a four-day period, a once-in-a-millennium event. 

In places where the watersheds of many rivers begin in the Appalachian mountain range, both flow and elevation reached record levels. In Asheville, for example, the level of the French Broad River rose to a peak of 24.7 feet (7.5 meters), breaking the record set during an infamous flood in 1916.

On October 11, after analyzing Hurricane Milton, the WWA confirmed that the same effects noted for Hurricane Helene were behind the exceptional strength of the second hurricane to hit Florida's west coast in less than two weeks.